Pond Design and Construction

A shrimp pond should be designed according to the characteristics of the selected site and the culture system. There is no unique design, but optimum and functional farm layout plan and design should be based on the physical and economic conditions prevailing in the locality.

Culture systems

There are three types of shrimp culture being practiced in most countries.

Traditional/Extensive Cultures

The ponds have irregular shapes and sizes, mostly 1.5 ha and bigger with a peripheral ditch or canal of 4-10 m wide and 40-80 cm deep. The pond bottom may not be properly leveled, but tree stumps are usually removed, although this is not required. Ponds are normally filled with gravity flow water during the high tide period with natural seeds and left for 60-90 days, without additional seed stocking and feeding. Stocking density in this type of culture is 0.5-5.0 pcs/m2. These ponds are normally partially harvested.

Semi-Intensive Culture

Ponds of 1-1.5 ha in size and are constructed with dikes to hold the water 1-1.5 m deep. PL are stocked into the pond at 10-15 PL/m2 and fed with commercial diets and/or fresh diets. The shrimp are harvested at 90-120 days after stocking.

Intensive Culture

The ponds are usually of 0.5-1 ha in size and are designed to keep the water at 1.5-2.0 m deep. A reservoir of at least 30 % of the pond area is usually required. High stocking density of 25-60 PL/m2 with feeding rate of 4-6 times daily and strong aeration is maintained.

Open System

This system requires a high supply of good quality water because it needs a water, exchange of more than 20% of the total pond volume at one time, in order to reduce pond wastes and the density of the plankton. Seed can be stocked up to 60 PL/m2 and will grow to 25-35 grams within 120 days. The open system has recently become less favorable to farmers since the environmental conditions, especially the quality of water, tend to deteriorate with time.

Re-circulation System

In order to avoid deterioration of the environmental conditions, several advanced and company run farms have adopted the re-circulation system to minimize contact with poor quality water from outside the farm. However, the farm must devote 40-50 % of the area for the construction of water storage/reservoir, sedimentation pond, treatment pond and drainage canals. To operate the system, cleaned seawater is initially pumped into the pond and kept within the system. During the culture period, the effluent from culture pond is drained into the sedimentation pond, treated with chemicals and pumped into the reservoir for re-supply to culture ponds. The stocking density for this system generally varies between 30-50 PL/m2 and the culture period is between 110-130 days.

Minimal Water Exchange System

The majority of small farms cannot support space for construction of the water treatment pond and reservoir as in the case of the re-circulation system. To reduce contact with the water from outside the farm, the minimal water exchange system or closed pond system is practiced in some countries, particularly in Thailand. The system involves filling up the pond with cleaned seawater, treating it with chemicals to eradicate predators and competitors. Then the shrimps are stocked up to 30 PL/m2 and cultured for a period of less than 100 days to attain the average weight of 10-20 gm. Since the system does not require water exchange, but maintains the water level in the pond by replacing the water loss due to evaporation and seepage with seawater or freshwater, it can be operated anywhere, even in the inland area where seawater is not easily accessible. The disadvantages of this system are that it requires low stocking density and high efficient water and waste management. However, it is suitable for production of small size shrimp because the culture period is limited.

Farm Design

An extensive shrimp farm should be of the size 0.4 - 0.5 ha and preferably drainable from the management point of view. The ponds generally should have concrete dikes, elevated concrete supply canal with separate drain gates and adequate life supporting devices like generators and aerators. The design, elevation and orientation of the water canals must be related to the elevation of the area with particular reference to the mean range of tidal fluctuation. The layout of the canals and dikes may be fitted as closely as technically possible to existing land slopes and undulation for minimizing the cost of construction.

Water Supply System

A shrimp pond is filled with water mostly by pumping. The pumps should be installed at locations where they can obtain water from the middle of the water column with least sedimentation and pollution. The pumps and inlet canal should be large enough to allow the ponds or the reservoir to be filled within 4-6 hrs. A screen should be installed at the inlet canal prior to the pumps to prevent clogging at the inlets.

Reservoir

A reservoir is important for the control of pond environment and storage of water supply when the water quality is inconsistent or the supply is intermittent. It is recommended that the area of a reservoir within a farm should be about 30% of the total farm area in order to hold a sufficient volume of the water supply. Some farms may use part of the reservoir for sedimentation purpose where biological filter feeding organisms are stocked. The reservoir must have an outlet that can allow total drainage.

Supply Canals

An intensive shrimp farm should have a water supply canal to convey the water from the reservoir to the ponds by gravity or pumping. The size of the supply canal will depend on the size of the culture pond, the efficiency of the pump and the required water exchange rate.

Ponds

A well-designed pond will facilitate the management of water exchange, harvesting of the product, waste collection and elimination, and feeding.

(i) Shape

The shapes of pond that are found to be effective for shrimp culture are rectangular, square and circular. A well-designed pond is one that would allow circulation of the water such that wastes will be accumulated at the center of the pond. Some farmers improve the water movement in the square and rectangular ponds by making the corners of the pond rounded through addition of soil.

(ii) Size

Smaller ponds are easier to manage but the construction and operation can be costly. Ponds of 0.5-1.0 ha. are commonly used in intensive culture and 1-2 ha for semi-intensive culture.

Dikes

Earthen dikes, with or without lining, are found to be the most economical. Dikes should be designed to impound higher than 1 m depth of water and must be high enough to prevent flooding during the rainy seasons and the highest high tide. The slope of the dike depends on the nature of the soil. A slope of not less than 1:1.5 is normally used in the sandy soil area to avoid erosion and 1:1 is used for clay soils. One must be aware that shallow slopes will encourage the growth of benthic algae which will impair the quality of the water in the pond. Some dikes in a farm may be wider than the others to provide space for the access road, storage, electricity and aerators.

Pond Lining

Lining materials are used in pond where the soil contains a high percentage of sand, and organic matter and is acidic in nature. Lining can reduce erosion, water seepage, waste accumulation in the soil and the leaching of ammonia, hydrogen sulfide, acidic compounds, iron and other potentially stressful compounds into the ponds. The lining also allows easy removal of wastes from the feeding areas, reducing the time and costs to clean the ponds between cycles. Several lining materials are currently available. The economic life of liners varies according to the maintenance and the duration of exposure to sunlight.

Among the liners, laterite soil is less expensive and commonly used in shrimp farms. However, laterite soil liner may allow the penetration of wastes and requires effective cleaning up. Pond liners with PVC plastic sheeting and geotextiles can reduce the cost for aeration and cleaning up due to the easy movement of wastes and uneaten food on the smooth surface. The disadvantages of PVC plastic and geotextile-lined ponds are difficulties in maintaining plankton bloom within the first month of culture, problem of tears and the floating of the liner if the water and gas accumulate underneath them.

Gates for Inlet and Outlet

Each shrimp pond should have at least one gate for filling and draining water. However, a typical pond of 0.5-1 ha usually consists of two gates having similar structure for the inlet and outlet gates. The size of the gate is dependent on the size of the pond, but must allow the pond to be filled or drained within 4-6 hrs. Gates of 0.5-1.0 m wide are usually constructed, since gates wider than 1 m will cause difficulty in screening and will allow strong currents which will cause erosion of the soil. The position of the outlet should be at the lowest point of the pond with a gradual slope of 1:200 from the inlet to allow total drainage of the pond during harvesting.

The conventional gates constructed at the side of the pond should have a double screen, with fine a mesh for the initial period of culture and a coarser one for a later period. Some farmers may place both meshes in a single frame and cut out the finer mesh when the size of the shrimps are larger than the opening of the coarser mesh.

Central Drain

This has been employed in some farms and consists of perforated pipes laid horizontally at the center of the pond and connected to a pipe leading to the outlet. A screen of small mesh size is used to cover the drain for the first 50 days of culture and is removed to allow for easy removal of water when the shrimps are larger than the diameter of the pipe. This method has the advantages in that it can remove the waste and clean the pond bottom any time throughout the culture period.

Drainage Canal and Sedimentation Pond

The drainage canal of a shrimp pond should be at least 50 cm lower than the lowest point of the pond to allow drainage by gravity. The effluent will be drained into a sedimentation pond to settle the particulate wastes before water is pumped into the reservoir or released out of the farm. It is recommended that the sedimentation pond should be approximately 5-10% of the culture area and should be deep enough to prevent mixing and re-suspension of the wastes. Baffles or soft walls made of fine mesh net or plastic sheeting supported by stakes driven into the pond bottom, may be constructed in the sedimentation pond to decrease the velocity of water and increase the retention time which will enhance the settlement of the wastes. The wastes in the sedimentation pond should be removed periodically and discharged into the waste dumping area.

Waste Dumping Area

A shrimp farm should provide 5-10% of the area for dumping of the wastes. Wastes from the pond must be collected carefully and dumped into this area without discharging to nearby areas, which will contaminate the natural resources.

Buildings

Accommodation, storage, shop and guardhouses may be built in the farm as required. It is advised that accommodation for workers should be set up at various points around the farm for security purposes and to allow the ponds to be adequately monitored.

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(Source:Photos courtesy: Shrimp Health Management Extension Manual- MPEDA & NACA

www.fishdb.sinica.edu.tw)